1. Field of the Invention
The invention relates to an optical analysis device according to the principle of radiation absorption with a housing with at least one radiation-permeable housing element, with at least a first radiation source and a first reflector assigned to it and a second radiation source and a second reflector assigned to it, with at least a first detector and a second detector and with an external reflector located outside the housing, the radiation sources and the detectors being located within the housing, an absorption space being formed by the external reflector and the radiation-permeable housing element, a measurement beam emitted by the first radiation source and the first reflector after reflection on the external reflector re-entering the housing, and a reference beam being emitted by the second radiation source and the second reflector.
2. Description of Related Art
Optical analysis devices of the type under consideration have long been known in various embodiments and exploit the effect of radiation absorption by matter which is penetrated by electromagnetic radiation. The electromagnetic radiation is generally broadband and covers at least the frequency range in which the substance to be detected acts to absorb radiation. Each substance to be detected shows a characteristic absorption spectrum which is characterized in that the emitted electromagnetic radiation, after passing through the absorption space in which the substance to be detected is located, is relatively strongly attenuated in certain, generally narrowband absorption regions relative to the radiated power of adjacent frequency ranges. While the substances present can be identified via recording of an absorption spectrum, it is also possible to draw conclusions about certain concentrations of the substance via the intensity of the relative attenuation in the absorption range.
Nondispersive infrared analysis is used especially for the detection of gases and does not involve spectral decomposition of the emitted electromagnetic radiation. For nondispersive spectrometers, selective detectors are used which are sensitive only in a limited radiation range, specifically in the range in which the substance to be detected absorbs radiation. The selectivity of the filters used is often dictated by the interference filters connected upstream of the detectors. The detectors are generally pyroelectric detectors or thermocouples interconnected to form thermopiles.
It can be easily imagined that, when using only one detector, almost no conclusions can be drawn about the actual concentration of the substance which is to be detected, if it is possible for attenuation of the measurement means to be caused in some other way, for example, by the presence of interfering gases and other contaminants in the absorption space. To the same degree, for example, ageing-induced intensity attenuation of the radiation source also cannot be detected with only one detector. To compensate for these effects, use of at least two detectors is therefore known in the prior art, of which one detector is sensitive in the absorption range of the substance to be detected and the other detector is sensitive in the frequency range in which absorption by other substances is not possible (reference detector and measurement detector). Certain effects which adulterate the measurement can be compensated for by the signal obtained from the measurement detector being referenced to the detector obtained from the reference detector.
For example, it is known that the radiation emitted by the radiation source can be divided by mirrors into a measurement beam and a reference beam, the measurement beam and the reference beam being detected by two different detectors. By using only one radiation source, for example, the sensitivity drift of individual detectors—for example, caused by ageing or temperature dependency—cannot be compensated for, and thus, has an effect on the final measurement signal (German Patent DE 44 37 188 C2).
German Patent DE 197 13 928 C1 and corresponding U.S. Pat. No. 5,923,035 disclose an optical analysis device of the type under consideration, in which there is a beam splitter in the housing which deflects or transmits both the measurement beam and also the reference beam partially in the direction of the first detector and of the second detector. Between the beam splitter and the first detector and the second detector, a respective concentrator is connected which is used to homogenize the measurement beam or reference beam. On the one hand, it is disadvantageous in this construction that the beam splitter can cause temperature-dependent asymmetrical intensity distributions or intensity losses in the measurement and reference beam which become part of the measurement signal in a manner which cannot be compensated. Furthermore, by using concentrators, the angle of incidence of the measurement and reference beam on the respective downstream detector or on the interference filter upstream of the detector is necessarily reduced, by which its bandpass characteristic becomes more broadband, and thus, less gas-specific. Moreover, both the measurement beam and also the reference beam are greatly attenuated by a plurality of reflections; this has an adverse effect on the sensitivity of the optical analysis device.
One underlying problem in the use of optical analysis devices of the type under consideration here is that the spectral absorption capacity of substances is very different in its absorption range. Some gases absorb only very weakly and only in a very narrow wavelength range. In order to achieve good detection sensitivity, for only weakly absorbing components compared to more strongly absorbing components a longer absorption distance is necessary to obtain comparable signals at the same concentration of the substance. Simple matching of the known optical analysis devices to substances which absorb in different degrees or to different sensitivity ranges is not easily possible in the analysis devices underlying the invention.